Shear wave elastrography method and apparatus for imaging an anisotropic medium

ABSTRACT

A shear wave elastography method for imaging an observation field in an anisotropic medium, including an initial ultrasonic acquisition step during which initial physical parameters are acquired in at least one region of interest; a spatial characterization step during which a set of spatial characteristics of the anisotropic medium is determined on the basis of the initial physical parameter; an excitation substep during which an shear wave is generated inside the anisotropic medium on the basis of the set of spatial characteristics; and an observation substep during which the propagation of the shear wave is observed simultaneously at a multitude of points in the observation field.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/539,170 filed on Jun. 23, 2017, which is the national phase of PCTInternational Application No. PCT/IB2014/003123 filed on Dec. 24, 2014,the contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to imaging methods and apparatuses usingshear waves, more precisely to shear wave elastography method andapparatuses for imaging anisotropic media.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 7,252,004 describes an example of a shear waveelastography method for imaging a viscoelastic medium. While the methodof U.S. Pat. No. 7,252,004 gives full satisfaction when the viscoelasticmedium is homogeneous, the reproducibility and reliability of images andmeasurements is not optimal when the viscoelastic medium is ananisotropic medium containing fibers.

In fact, measurements and images obtained with shear wave elastographyare computed from the observed displacement and/or deformation of theviscoelastic medium submitted to a shear wave.

Unfortunately, shear wave propagates differently in anisotropic mediumcontaining fibers compared to homogeneous medium since their propagationparameters depend not only of the physical characteristics of the mediumbut also of the relative angle of the shear wave front with fibersdirections.

Thus, depending on the relative angle of the shear wave propagationdirection with fibers directions, the measured value of the propagationparameters of the shear wave can vary and gives non reliable and nonreproducible measurements and images to the shear wave elastography.

The instant invention has notably for object to improve the situation.

SUMMARY OF THE INVENTION

To this aim, according to the invention, such a shear wave elastographymethod for imaging an anisotropic medium comprises

a) an initial ultrasonic acquisition step during which at least oneinitial physical parameter is acquired in at least one region ofinterest in the anisotropic medium;

b) a spatial characterization step during which a set of spatialcharacteristics of the anisotropic medium is determined on the basis ofthe initial physical parameter; and

c) a shear wave imaging step comprising:

-   -   c1) an excitation substep during which a shear wave is generated        inside the anisotropic medium on the basis of said set of        spatial characteristics; and    -   c2) an observation substep during which the propagation of said        shear wave is observed simultaneously at a multitude of points        in the observation field.

In some embodiments, one might also use one or more of the followingfeatures:

-   -   the set of spatial characteristics of the anisotropic medium        comprises at least one of the following: a direction, spatial        angle or spatial position of anisotropic features in the at        least one region of interest, and a preferred excitation spatial        direction in the at least one region of interest;    -   the initial physical parameter is an image of the at least one        region of interest in the anisotropic medium acquired using        B-mode ultrasonic imaging;    -   the initial ultrasonic acquisition step comprises a shear wave        imaging step comprising:

an excitation substep during which an shear wave is generated inside theanisotropic medium with at least one shear wave direction, and

an observation substep during which the propagation of said shear waveis observed simultaneously at a multitude of points in the at least oneregion of interest to acquire an initial physical parameter;

-   -   the initial ultrasonic acquisition step comprises a plurality of        shear wave imaging steps associated with a plurality of shear        wave directions and with a plurality of initial physical        parameters acquired in at least one region of interest in the        anisotropic medium, each shear wave imaging step of the        plurality of shear wave imaging steps comprising an excitation        substep during which a shear wave is generated inside the        anisotropic medium with an associated shear wave direction of        the plurality of shear wave directions, and an observation        substep during which the propagation of said shear wave is        observed simultaneously at a multitude of points in the at least        one region of interest to acquire an associated initial physical        parameter of the plurality of initial physical parameters;    -   said initial physical parameters are images of the region of        interest in the anisotropic medium acquired using shear wave        imaging;    -   said initial physical parameters are shear wave propagation        parameters, acquired in the at least one region of interest,        using shear wave imaging;    -   said shear wave propagation parameters are selected from shear        wave speed, shear modulus μ, Young's modulus E, shear elasticity        μ1, shear viscosity μ2;    -   during the initial ultrasonic acquisition step, at least two        initial physical parameters are acquired, respectively        associated to at least two distinct regions of interest in the        anisotropic medium, and, during the spatial characterization        step, at least two sets of spatial characteristics are        determined, respectively on the basis of the at least two        initial physical parameters, and respectively associated to the        at least two distinct regions of interest in the anisotropic        medium, and, during the shear wave imaging step, during the        excitation substep, at least two shear waves are generated        inside the anisotropic medium, respectively on the basis of the        at least two sets of spatial characteristics, and, during the        observation substep, the propagation of said at least two shear        waves is observed simultaneously at a multitude of points in the        observation field;    -   the spatial characterization step comprises extracting a set of        spatial characteristics by performing features detection on at        least one image of the anisotropic medium acquired during the        initial ultrasonic acquisition step;    -   the spatial characterization step comprises comparing shear wave        propagation parameters of the plurality of shear wave        propagation parameters acquired during the initial ultrasonic        acquisition step to determine a preferred excitation spatial        direction in the anisotropic medium;    -   the spatial characterization step comprises displaying an image        of the anisotropic medium acquired during the initial ultrasonic        acquisition step to a user using a display device connected to a        central processing unit, said user indicating spatial        characteristics of the anisotropic medium using an input device        connected to said central processing unit;    -   said user indicates spatial characteristics of the anisotropic        medium by moving, using said input device, a position of a        virtual line displayed above said image of the anisotropic        medium on said display device, said line being indicative of a        spatial characteristic of the anisotropic medium;    -   said user indicates spatial characteristics by measuring, on        said image of the anisotropic medium displayed on the display        device, a numerical value of a spatial characteristic of the        anisotropic medium using conventional angle measurement tools        provided by an ultrasound system, said user then entering said        numerical value in the central processing unit using the input        device;    -   the shear wave is generated by emitting at least one focused        ultrasound waves in the anisotropic medium using an array of        transducers controlled by the central processing unit, the        location of the focal points of said focused ultrasound waves        and the timing of said focused ultrasound waves being determined        by the central processing unit on the basis of the set of        spatial characteristics of the anisotropic medium;    -   the location of the focal points and the timing of the plurality        of focused ultrasound waves generating the shear wave are        determined so that a wave front of said shear wave is        substantially perpendicular to at least one of: a direction of        anisotropic features in the anisotropic medium and a preferred        excitation spatial direction in the anisotropic medium;    -   the location of the focal points and the timing of the plurality        of focused ultrasound waves generating the shear wave are        determined so that a propagation direction of said shear wave is        substantially aligned with at least one of: a direction of        anisotropic features in the anisotropic medium and a preferred        excitation spatial direction in the anisotropic medium;    -   the observation substep comprises the operations of:

c2-1) causing an array of transducers that are controlled independentlyof one another to emit into the anisotropic medium a succession ofultrasound waves with spatial coverage and timing adapted so that saidultrasound waves exhibit at least partial spatial and temporal overlapwith the propagating shear wave in the observation field, and

c2-2) causing sound signals received from the anisotropic medium to bedetected and recorded in real time by said array of transducers, saidsignals comprising echoes generated by the ultrasound waves interactingwith scatterers in said anisotropic medium,

the shear wave imaging step c) further comprising at least oneprocessing substep c3) during which:

c3-1) the sound signals received successively from the anisotropicmedium during operation c2-2) are processed in order to determinesuccessive propagation images of the shear wave, and

c3-2) at least one movement parameter of the anisotropic medium isdetermined at different points of the observation field;

-   -   during the initial ultrasonic acquisition step, a shear wave        propagating along at least two shear wave directions is        generated inside the anisotropic medium, and

during the processing substep, sound signals received from theanisotropic medium are filtered according to the at least two shear wavedirections to determine said at least one initial physical parameter;

-   -   said movement parameter is a displacement of the anisotropic        medium;    -   at said operation c2-1), said unfocused ultrasound compression        waves are emitted at a rate of at least 300 shots per second;    -   the focused ultrasound wave emitted during excitation substep        presents a frequency f lying in the range 0.1 MHz to 100 MHz,        and is emitted for a duration of k/f seconds, where k is an        integer lying in the range 50 to 5000 and f is expressed in Hz;    -   the focused ultrasound wave emitted during excitation substep        presents a frequency lying in the range 0.5 MHz to 15 MHz and is        emitted during a succession of emission periods separated by        rest periods, the emission periods following one another at a        rate lying in the range 10 to 1000 emissions per second;    -   the focused ultrasound wave emitted during excitation substep        c1) is a linear combination of two monochromatic signals having        respective frequencies f1 and f2 such that 20 Hz≤|f1−f2|≤1000        Hz;    -   a focused ultrasound wave emitted during excitation substep is        focused simultaneously on a plurality of focal points;    -   image processing substep is followed by a mapping substep during        which, on the basis of variation in the movement parameter over        time, at least one shear wave propagation parameter is        calculated at at least some points of the observation field in        order to determine a map of said propagation parameter in the        observation field;    -   the shear wave propagation parameter which is calculated during        mapping substep is selected from shear wave speed, shear        modulus, Young's modulus, shear wave attenuation, shear        elasticity, shear viscosity, mechanical relaxation time and the        inverse of local strain;    -   substeps are repeated successively while emitting different        plurality of focused ultrasound waves during successive        excitation substeps, and then combining the maps obtained during        the successive mapping substeps in order to calculate a        combination map of the observation field;    -   steps b) and c) are reiterated at least once, a map of a        propagation parameter in the observation field acquired during        step c) at iteration n being used as initial physical parameter        for step b) at iteration n+1.

The invention also has as an object, an imaging apparatus forimplementing a shear wave elastography method as detailed above forimaging an observation field in an anisotropic medium, the apparatuscomprising an array of transducers that are controlled independently ofone another by at least one electronic central unit adapted:

-   -   to acquire at least one initial physical parameter in at least        one region of interest in the anisotropic medium;    -   to determine a set of spatial characteristics of the anisotropic        medium on the basis of the initial physical parameter;    -   to cause an shear wave to be generated inside the anisotropic        medium on the basis of said set of spatial characteristics; and    -   to observe the propagation of said shear wave simultaneously at        a multitude of points in the observation field.

With these features, the shear wave characteristics and in particularthe wave front and the propagation direction of the shear wave can bedetermined on the basis of the spatial characteristics of theanisotropic medium. This strongly improves the quality, reliability andreproducibility of the images and measurement obtained by shear waveelastography and imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention appear from the followingdetailed description of one embodiment thereof, given by way ofnon-limiting example, and with reference to the accompanying drawings.

In the drawings:

FIG. 1 is a diagrammatic view of a shear-wave imaging device in anembodiment of the invention;

FIGS. 2a, 2b and 2c are schematic views of several shear waves generatedin an anisotropic medium by different plurality of focused ultrasoundwaves.

DETAILED DESCRIPTION

The apparatus 1 shown on FIG. 1 is adapted for imaging an observationfield 2 a in an anisotropic medium 2.

The anisotropic medium 2 contains anisotropic features 3 which presentidentifiable spatial extension directions 3 a. Anisotropic features 3can be for instance fibers, tissue interfaces or anisotropic tissues,vessels, nerves, anisotropic constituents such as elongated cells ororganelles and more generally any spatially oriented constituent of atissue, in particular a living tissue.

Anisotropic medium 2 can for instance be a part of a living patientcontaining muscles or tendons. Such tissues contain anisotropic features3 in the form of fibers 3, for instance muscles fascicles surrounded byperimysium, or arrays of collagen fibers in tendons, ligaments andfasciae. Such fibers 3 are strongly anisotropic and present identifiablespatial extension directions 3 a, thus rendering the medium 2anisotropic. Some fibers 3 might also be grouped in bundles of fibersusually extending along parallel directions 3 a.

More precisely, the apparatus 1 is used to perform shear waveelastography of the anisotropic medium 2 to obtain an image or ameasurement of shear wave propagation parameters in the observationfield 2 a.

A conventional shear wave elastography measurement or imaging is carriedout by:

-   -   having a mechanical shear wave 14 propagates through the        anisotropic medium 2, in particular through the observation        field 2 a; and    -   observing the propagation of this shear wave 14 in the        observation field 2 a through reflexion of ultrasonic waves on        scatterers 2 b which are reflective for the ultrasound waves and        are naturally contained in biological tissues.

The particles 2 b may be constituted by any non-uniformity in theanisotropic medium 2, for instance particles of collagen or moregenerally any inhomogeneity having a density that differs for thedensity of the surrounding medium.

Shear wave elastography measurements and images are then processed basedon the observed displacement and/or deformation of tissues whenpenetrated by the shear wave.

As already mentioned, shear wave propagates differently in anisotropicmedia compared to homogeneous media.

In particular, the propagation of the shear wave depends not only on thephysical characteristics of the medium 2 but also on the angle of theshear wave front, or of the shear wave propagation direction 14 a, withfavored directions 3 a in the anisotropic medium 2.

Consequently, depending for instance on the relative angle of the shearwave propagation direction 14 a with anisotropic features 3, themeasured value of the propagation parameters of the shear wave 14 canvary thus giving non reliable and non reproducible shear waveelastography measurements and images.

The structure and general way of operation of a shear wave imaging stepc) such as the one performed by apparatus 1 has already been describedin details in document U.S. Pat. No. 7,252,004 and will be shortlyrecalled hereafter.

The apparatus 1 may include for instance:

-   -   an ultrasound transducer array 4, for instance a linear array        typically including n ultrasonic transducers T₁-T_(n) juxtaposed        along an axis as already known in usual echographic probes (the        array 4 is then adapted to perform a bidimensional (2D) imaging        of the observation field, but the array 4 could also be a        bidimensional array adapted to perform a 3D imaging of the        observation field); the number n of transducers is more than 1,        for instance a few tens (e. g. 100 to 300); the transducers        T₁-T_(n) deliver ultrasound wave pulses, which pulses are of the        type commonly used in echography, for example having a frequency        lying in the range 0.5 MHz to 100 MHz, and preferably in the        range 0.5 MHz to 15 MHz, e.g. being about 2.5 MHz;    -   an electronic bay 5 controlling the transducer array 4 and        acquiring signals there from;    -   a microcomputer 6 for controlling the electronic bay 5 and        viewing ultrasound images obtained from the electronic bay, said        computer 6 including a display unit 6 a, for instance a screen,        and input devices 6 b such as a keyboard, a mouse or other user        interfaces.

The electronic bay 5 and the microcomputer 6 will be referred herein asthe control system of the apparatus 1. Such control system might beconstituted of more than two devices, or by one single electronic devicecould fulfill all the functionalities of the electronic bay 5 and of themicrocomputer 6.

The electronic bay 5 may include for instance:

-   -   n analog/digital converters 7 (E₁-En) individually connected to        the n transducers (T₁-Tn) of the transducer array 4;    -   n buffer memories 8 (M₁-Mn) respectively connected to the n        analog/digital converters 7;    -   a central processing unit 9 (CPU) communicating with the buffer        memories 8 and the microcomputer 6;    -   a digital signal processor 11 (DSP) connected to the central        processing unit 9;    -   a memory 10 (MEM) connected to the central processing unit 8.

The transducers T₁-T_(n) are controlled independently of one another bythe central processing unit 9. The transducers T1-Tn can thus emitselectively:

-   -   an unfocussed ultrasound wave;    -   or a focused ultrasound wave that is focused on one or more        points of the observation field 2 a.

The wording “unfocussed ultrasound wave” as understood herein means anyunfocussed wave illuminating the entire observation field 2 a, forinstance:

-   -   an ultrasound compression wave that is “plane” (i.e. a wave        whose wave front is rectilinear in the X,Y plane), or any other        type of unfocused wave;    -   a wave generated by causing random sound signals to be emitted        by the various transducers T₁-Tn;    -   a diverging wave, for instance a spherical waves;    -   a wave focused simultaneously on several focal points;    -   a weakly focusing waves (known as “fat” transmit focusing: ratio        Focal distance/Aperture>2.5);    -   or more generally any kind of transmit waves that do not        correspond to conventional focusing using a single focal point        location and a ratio Focal distance/Aperture<2.5.

During operation of the apparatus 1, the transducer array 4 is placed incontact with the anisotropic medium 2, for instance with the skin of apatient.

The way of operation of the apparatus 1 is controlled by the controlsystem, i.e. the central processing unit 9 and/or the computer 6, whichare programmed for this way of operation. These two devices willhereafter be called the control system of apparatus 1 (of course, thecontrol system could be different from the particular example describedherein and in particular could be constituted by one single electronicdevice as recalled before, or by more than two electronic devices).

The operation of apparatus 1 to perform shear wave imaging will now bedescribed in relation with shear wave imaging step c) but also apply toembodiments of the initial ultrasonic acquisition step a) in whichinitial physical parameters are acquired by shear wave imaging orelastography as described in further details hereafter.

The control system 6, 9 of the apparatus 1 is programmed to performseveral substeps in succession, starting with an excitation substep c1)during which the control system 6, 9 causes a shear wave 14 to begenerated in the observation field 2 a by causing at least one focusedultrasound wave 13, focused on a focal point 13 a of the anisotropicmedium 2, to be emitted by the array 4 (this focussed wave 13 may beemitted by all or part of the transducers T₁-Tn);

In particular and as it will be further detailed below, the focusedultrasound waves 13 emitted during excitation substep c1) may be focusedon a plurality of points 13 a simultaneously or at different times sothat the shear wave 14 as generated presents a desired wave shape.

It is thus possible to generate a shear wave 14 that is plane, or on thecontrary a shear wave that is focused) and illuminates desired zones inthe observation field 2 a of the anisotropic medium 2.

In particular, the shear wave 14 can present a shear wave front and ashear wave direction 14 a that can be controlled by the position of thefocal points 13 a and the timing of the emission of the focusedultrasound waves 13.

With reference to FIG. 2a , a specific shear wave 14 can thus begenerated by the emission of several focused ultrasound waves 13simultaneously or within a short period of time, said ultrasound waves13 having focal points 13 a aligned along a line in the anisotropicmedium 2.

The resulting shear wave 14 has a shear wave front that is substantiallyparallel to the alignment line of the focal points 13 a and apropagation direction 14 a that is substantially perpendicular to saidline.

With reference to FIG. 2b , a specific shear wave 14 is illustrated thatis generated by the emission of several focused ultrasound waves 13 witha longer period of time separating each emission, said ultrasound waves13 having focal points 13 a aligned along a line in the anisotropicmedium 2.

The resulting shear wave 14 has a shear wave front with a higher anglerelative to the alignment line of the focal points 13 a than the aboveshear wave illustrated on FIG. 2 a.

Referring now to FIG. 2c , another specific shear wave 14 is illustratedthat is generated by the emission of several focused ultrasound waves 13having focal points 13 a aligned along a different line in theanisotropic medium 2, said line being inclined relatively to a verticaldirection perpendicular to the array 4.

FIGS. 2a, 2b and 2c thus illustrates some of the various shear wave 14,that can be generated by the control system 6, 9 of the apparatus 1 byvarying the position of the focal points 13 a of the focused ultrasoundwaves 13 and the timing of the emission of the focused ultrasound waves13.

Other types of shear wave 14 can also be generated, for instance, byvarying the position of the focal points 13 a of the focused ultrasoundwaves 13 and the timing of the emission of the focused ultrasound waves13, the control system 6, 9 of the apparatus 1 may generate a shear wave14 propagating along two or more shear wave directions. By “two shearwave directions”, it is understood that said shear wave directions arenot collinear.

The focused ultrasound wave emitted during the excitation substep c1)may be a monochromatic wave of frequency f lying in the range 0.5 MHz to15 MHz, for example being equal to about 2.5 MHz, which is emitted for aduration of k/f seconds, where k is an integer lying in the range 50 to5000 (e.g. being about 500) and f is expressed in Hz. Such a wave maypossibly be emitted during a succession of emission periods separated byrest periods, the emission periods following one another at a rate lyingin the range 5 to 1000 emissions per second.

In a variant, the focused ultrasound wave emitted during excitationsubstep c1) is a linear combination (in particular a sum) of twomonochromatic signals of respective frequencies f1 and f2 such that 20Hz≤|f1−f2|≤1000 Hz, thus producing an amplitude modulated wave having amodulation frequency |f1−f2|.

The apparatus 1 then performs an observation substep c2) during whichthe propagation of the shear wave 14 is observed simultaneously at amultitude of points of the observation field 2 a), this observation stepcomprising the following operations:

c2-1) the control system 6, 9 causes the array 4 to emit into theanisotropic medium a succession of unfocused ultrasound compressionwaves (these unfocussed waves may be emitted by all or part of thetransducers T₁-T_(n)) at a rate of at least 300 shots per second, forinstance at least 500 shots/s (the focusing and the timing of thefocussed ultrasound wave emitted in step a), and the timing of saidunfocused ultrasound waves are adapted so that at least some of saidunfocused ultrasound waves reach the observation field during thepropagation of the shear wave through the observation field);

c2-2) the control system 6, 9 causes the array 4 to detect sound signalsreceived from the anisotropic medium 2 (this detection can be carriedout by all or part of the transducers of the array 4), said signalscomprising echoes generated by the unfocused ultrasound compression waveinteracting with scatterers 2 b in the observation field, these echoescorresponding (directly or indirectly) to successive images of thedisplacement of the anisotropic medium 2; the detected signals arerecorded in real time in the buffer memories M₁-Mn;

The apparatus 1 then performs at least one processing substep c3) duringwhich:

c3-1) the control system 6, 9 processes the successive sound signalsreceived from the anisotropic medium 2 during operation c2-2) in orderto determine successive propagation images; and

c3-2) the control system 6, 9 determines at least one movement parameterfor the anisotropic medium 2 at various points in the observation field2 a.

It should be noted that the above operation c3-1) could be omitted: moregenerally, the method of the invention does not require determiningpropagation images, and the control system 6, 9 may determine saidmovement parameter by any other means.

During operation c2-1), which may last for example 0.1 to 180 s, it ispossible to emit unfocused ultrasound compression waves at a rate lyingin the range 500 to 10,000 shots per second, and preferably in the range1000 to 5000 shots per second (with this rate being limited by thego-and-return travel time for the compression wave through the patient'sbody 2: it is necessary for all of the echoes that are generated by thecompression wave to have been received by the probe 6 before a newcompression wave is sent).

In the embodiment wherein the shear wave, emitted during the initialultrasonic acquisition step, propagates along two or more shear wavedirections, sound signals received from the anisotropic medium may befiltered, during the processing substep, according to said shear wavedirections to determine said initial physical parameter.

In one embodiment, the sound signals received from the anisotropicmedium are filtered to determine two or more initial physical parametersrespectively associated to the two or more shear wave directions.

Such a filtering operation may for instance be a temporal or spatialfiltering of the successive sound signals received from the anisotropicmedium 2 during operation c2-2) or of the movement parameters determinedduring operation c3-2).

Each unfocused ultrasound compression wave propagates through thepatient's body 2 at a propagation speed that is much higher than that ofshear waves (e.g. about 1500 m/s in the human body), and interacts withthe reflecting particles 2 b, thereby generating echoes or otheranalogous disturbances in the signal that are known in themselves underthe name “speckle noise” in the field of echography.

The speckle noise is picked up by the transducers T₁-T_(n) duringsubstep b2), after each shot of an unfocused ultrasound compressionwave. The signal s_(ij)(t) as picked up in this way by each transducerT_(i) after shot No. j is initially sampled at high frequency (e.g. 30MHz to 100 MHz) and digitized (e.g. on 12 bits) in real time by theanalog/digital converter E_(i) corresponding to transducer T_(i).

The signal s_(ij)(t) as sampled and digitized in this way is thenstored, likewise in real time, in a the buffer memory M_(i)corresponding to the transducer T_(i).

By way of example, each memory Mi may present a capacity of about 128megabytes (MB), and contains all of the signals s_(ij)(t) received insuccession for shots j=1 to p.

In deferred time, after all of the signals s_(ij)(t) corresponding tothe same propagation of a shear wave have been stored, the central unit9 processes these signals (or have them processed by another circuitsuch a summing circuit, or the computer 6 may process the signalsitself) using a conventional path-forming step corresponding to substepc1).

This generates signals S_(j)(x,y) each corresponding to the image of theobservation field after shot No. j.

For example, it is possible to determine a signal S_(j)(t) by thefollowing formula:

${S_{j}(t)} = {\sum\limits_{i = 1}^{n}{{\alpha_{i}\left( {x,y} \right)} \cdot {s_{ij}\left\lbrack {{t\left( {x,y} \right)} + {{d_{i}\left( {x,y} \right)}/V}} \right\rbrack}}}$

where:

-   -   s_(ij) is the raw signal perceived by the transducer No. i after        ultrasound compression wave shot No. j;    -   t(x,y) is the time taken by the ultrasound compression wave to        reach the point of the observation field having coordinates        (x,y), with t=0 at the beginning of shot No. j;    -   d_(i)(x,y) is the distance between the point of the observation        field having coordinates (x,y) and transducer No. i, or an        approximation to said distance;    -   V is the mean propagation speed of ultrasound compression waves        in the viscoelastic medium under observation; and    -   α_(i)(x,y) is a weighting coefficient taking account of        apodization relationships (in practice, in numerous cases, it is        possible to assume that α_(i)(x,y)=1).

The above formula applies mutatis mutandis when the observation field isthree-dimensional (with a two-dimensional array of transducers), withspace coordinates (x,y) being replaced by (x,y,z).

After the optional path-forming step, the central unit 9 stores in thecentral memory M, the image signals S_(j)(x,y) (or S_(j)(x) if the imagewould be in 1 dimension only, or S_(j)(x,y,z) in case of a 3D image),each corresponding to shot No. j. These signals may also be stored inthe computer 6 if the computer itself performs the image processing.

These images are then processed in deferred time in operation c3-2) bycorrelation and advantageously by cross-correlation either in pairs, orpreferably with a reference image, as explained in U.S. Pat. No.7,252,004. The above-mentioned cross-correlation can be performed, forexample, in the digital signal processor 11, or it may be programmed inthe central unit 9 or in the computer 6.

During this cross-correlation process, a cross-correlation function<S_(j)(x,y),S_(j+1)(x,y)> is maximized in order to determine thedisplacement to which each particle 2 b giving rise to an ultrasoundecho has been subjected.

Examples of such cross-correlation calculations are given in U.S. Pat.No. 7,252,004.

This produces a set of displacement vectors ū(r,t) generated by theshear waves in each position r of the observation field 2 a of theanisotropic medium 2 under the effect of the shear wave (thesedisplacement vectors may optionally be reduced to a single component inthe example described herein).

This set of displacement vectors is stored in the memory M or in thecomputer 6 and can be displayed, for example, in particular by means ofthe screen 4 a of the computer, in the form of a slow motion picture inwhich the values of the displacements are illustrated by an opticalparameter such as a gray level or a color level.

The propagation differences of the shear wave between zones havingdifferent characteristics in the anisotropic medium 2 can thus be seenclearly.

The motion picture of shear wave propagation can also be superposed on aconventional echographic image, which can also be generated by theapparatus 1 described above.

Furthermore, it is also possible to calculate, instead of displacements,the deformations of the anisotropic medium 2 for each of the points inthe observation field 2 a, i.e. vectors whose components are thederivatives of the displacement vectors respectively relative to thespace variables (X and Y coordinates in the example described). Thesedeformation vectors can be used like the displacement vectors forclearly viewing the propagation of the shear wave in the form of amotion picture, and they also present the advantage of eliminatingdisplacements of the transducer array 4 relative to a patient's bodyunder observation.

From the displacement or deformation fields, the computer 6 (or moregenerally the control system 6, 9) can advantageously then proceed witha map-making substep c4) during which, on the basis of the way in whichthe movement parameter (displacement or deformation) varies over time inthe field of observation X, Y (or X, Y, Z with a two-dimensional arrayof transducers), it calculates at least one propagation parameter of theshear wave, either at certain points (at least 1 point) in theobservation field 2 a as selected by the user acting on the computer 6,or else throughout the observation field 2 a.

The propagation parameter of the shear wave that is calculated duringthe map-making substep c4) is selected, for example, from amongst: theshear modulus μ, or Young's modulus E=3μ, or the propagation speed c_(s)of shear waves

$\left( {{c_{s} = \sqrt{\frac{E}{3\rho}}},} \right.$

where ρ is the density of the tissues), or the shear elasticity μ1, asexplained in more details in U.S. Pat. No. 7,252,004, or the inverse oflocal strain. Such propagation parameter is representative of theelasticity of the anisotropic medium constituting the observation field2 a.

This propagation parameter may be computed for instance by the computer6, repeatedly at several different instants, several times per second(e.g. at a rate of at least 5 times per second, e.g. at least 10 timesper second).

Preliminary to the shear wave imaging step c), the apparatus 1 performsseveral steps that will now be detailed further.

During an initial imaging step a), at least one initial physicalparameter is acquired for at least one region of interest 2 c in theanisotropic medium 2.

In a first embodiment, the initial physical parameter is acquired usingB-mode ultrasonic imaging. The initial physical parameter can be, inparticular, an image of the region of interest 2 c in the anisotropicmedium 2.

To this aim, the control system 6, 9 can perform a conventional B-modeultrasound image of the observation field 2 a using the transducer array4 in a standard ultrasound way. Standard ultrasound imaging consists ofan insonification of the medium with a cylindrical wave that focuses ona given point. Using the backscattered echoes of this singleinsonification, a complete line of the image is computed using a dynamicreceive beamforming process.

In a second embodiment of the invention, the initial physical parameteris acquired using shear wave elastography or imaging.

In this second embodiment, the initial physical parameter can be a shearwave propagation parameter or an image obtained by shear wave imaging.

The control system 6, 9 may in particular acquire the image byperforming a shear wave elastography or imaging similar to the shearwave imaging step c) described here-before.

Thus, the initial ultrasonic acquisition step may comprises a shear waveimaging step comprising:

a1) an excitation substep during which a shear wave is generated insidethe anisotropic medium with a shear wave direction; and

a2) an observation substep during which the propagation of said shearwave is observed simultaneously at a multitude of points in the at leastone region of interest to acquire an initial physical parameter.

Advantageously, the initial imaging step a), can comprise theacquisition of several initial physical parameters, each comprising stepa1) and step a2).

Thus several shear wave propagation parameters may be measured at one orseveral points of interest 2 b or within the region of interest 2 c ofthe anisotropic medium.

Several images of the region of interest of the anisotropic medium mayalso be acquired.

The shear wave propagation parameters may for instance be selected fromshear wave speed, shear modulus μ, Young's modulus E, shear elasticityμ1, shear viscosity μ2.

Then, in a spatial characterization step b), a set of spatialcharacteristics of the region of interest in the anisotropic medium isdetermined based on the initial physical parameter or the plurality ofinitial physical parameters.

The set of spatial characteristics may for instance comprises thedirection of anisotropic features 3 or the spatial angle of theanisotropic features 3 with a reference plane or line such as thedirection of extension of the transducer array 4, for instance thespatial angle of fibers direction 3 a with the direction of extension ofthe transducer array 4. The set of spatial characteristics may alsocomprises the spatial position or location of the anisotropic features 3in the anisotropic medium 2, in particular, the spatial position of saidanisotropic features in the bidimensional (2D) or tridimensional (3D)imaging of the region of interest. The set of spatial characteristicsmay also comprise a preferred excitation spatial direction in the regionof interest.

Advantageously, in a first embodiment of the invention, the initialphysical parameters are B-mode or shear wave images and the spatialcharacterization step b) can be performed automatically. In particular,step b) may thus comprise the extraction of a set of spatialcharacteristics by performing features detection on at least one imageof the observation field in the anisotropic medium acquired during theinitial ultrasonic acquisition step.

In a variant of this first embodiment of the invention, the spatialcharacterization step b) may be performed manually.

To this aim, a B-mode or shear wave image may be displayed on thedisplay device 6 a to be seen by a user, for instance a medicalpractitioner or an operator using apparatus 1.

Said user can then indicate spatial characteristics of fibers 3 usingthe input devices 6 b connected to said central processing unit 6, 9.

In a first variant of the invention, user indicates spatialcharacteristics of fibers 3 by moving the position of a virtual linedisplayed above the image of the anisotropic medium on said displaydevice, said line being indicative of a direction of fibers. The usercan for instance move said line using the input devices 6 b, forinstance the mouse and the keyboard.

In another variant, the user measures a numerical value of a spatialangle of fibers on the image of the anisotropic medium displayed on thedisplay device. The user can perform this measurement using for instanceconventional angle measurement tools provided on every standardultrasound system.

The user can then enter the measured numerical value in the centralprocessing unit using the input device in order to indicate spatialcharacteristics of fibers 3.

In a third embodiment, of the invention, the initial physical parametersare shear wave propagation parameters and the spatial characterizationstep b) may also be performed automatically as follow.

In this embodiment, the initial ultrasonic acquisition step a) mayadvantageously comprise a plurality of shear wave imaging stepsassociated with a plurality of shear wave directions and with aplurality of initial physical parameter acquired in at least one regionof interest in the observation field in the anisotropic medium.

More precisely, each shear wave imaging step is performed substantiallyas described here before in relation with shear wave imaging step c).Thus, each shear wave imaging step comprises first an excitation substepa1) during which a shear wave is generated inside the anisotropic mediumwith an associated shear wave direction of the plurality of shear wavedirections. Thus several shear waves are generated having distinct shearwave directions.

Each shear wave imaging step further comprises an observation substepa2) during which the propagation of the shear wave is observedsimultaneously at a multitude of points in the observation field toacquire an associated initial physical parameter, being advantageously ashear wave propagation parameter in this embodiment.

Following this initial ultrasonic acquisition step a), the spatialcharacterization step b) then comprises a comparison of the acquiredshear wave propagation parameters together in order to determine apreferred excitation spatial direction in the anisotropic medium.

For instance, when the acquired shear wave propagation parameters areshear wave speeds, the highest value of the acquired shear wave speedamong the plurality acquired shear wave speeds may advantageously becorrelated with an excitation shear wave direction having the bestalignment with the anisotropic features 3 among the plurality ofexcitation shear wave directions employed during the initial ultrasonicacquisition step a). It is thus possible to determine a preferredexcitation spatial direction in the anisotropic medium 2.

Step a) and b) may also be performed for two or more points of interestsand two or more regions of interest. This may be advantageous forinstance when the observation field of the anisotropic medium presentsseveral regions having distinct spatial properties. For instance, afirst region of the observation field may contain fibers oriented alonga first direction while a second region of the observation field maycontain vessels oriented along a second direction.

In this case, it is thus advantageous to define a first and a seconddistinct region of interest, respectively comprised within the first andthe second region of the observation field.

The set of spatial characteristics of the anisotropic medium determinedduring step b) may then comprises a first set of spatial characteristicsassociated with the first region of interest and a second set of spatialcharacteristics associated with the second region of interest.

More than two regions of interest may be defined depending on theanisotropic medium 2.

When the set of spatial characteristics has been determined, the controlsystem 6, 9 of the apparatus 1 can then generate a shear wave 14 adaptedto the spatial characteristics of the anisotropic medium 2 by varyingthe position of the focal points 13 a of the focused ultrasound waves 13and the timing of the emission of the focused ultrasound waves 13 asdetailed here-above in relation with substep c1).

In particular, the control system 6, 9 of the apparatus 1 can thengenerate an shear wave whose propagation direction is substantiallyaligned with the spatial extension directions 3 a of the anisotropicfeatures 3.

The control system 6, 9 of the apparatus 1 can also generate an shearwave whose shear wave front is substantially perpendicular to thespatial extension directions 3 a of the anisotropic features 3.

When more than two regions of interest are defined, the spatialcharacteristics of the shear wave, in particular the shear wavepropagation directions, may then be adapted for each regions ofinterest. Thus, a shear wave having at least two propagation directionsmay be generated. Such a shear wave is for instance a spherical wave andin particular may thus be a wave that is not a plane wave but a complexwave.

Alternatively, several shear waves may be emitted each being generatedon the basis of spatial characteristics associated with a region ofinterest.

The spatial characterization step b) and the shear wave imaging step c)may also be reiterated to further refine the set of spatialcharacteristics.

An image obtained during step c) of iteration n, for instance a map of apropagation parameter determined by a map-making substep c4), can thenbe used as the initial physical parameter for step b) at iteration n+1.

It should be noted that the method of the invention may further includea tracking step or substep for tracking deformations and displacement ofthe anisotropic medium 2 (in particular of anisotropic features 3) sothat the measurements of ultrasonic parameter are done at a samelocation within the anisotropic medium 2.

Besides, shear wave imaging as described above may be coupled withconventional ultrasound imaging provided in real time by the sameapparatus.

1. A shear wave elastography method for imaging an observation field inan anisotropic medium, the method comprising: a) an initial ultrasonicacquisition step of acquiring at least one initial physical parameter inat least one region of interest in the anisotropic medium; b) a spatialcharacterization step of determining a set of spatial characteristics ofthe anisotropic medium based on the initial physical parameter, said setof spatial characteristics comprising a direction or a spatial angle ofan anisotropic feature in the at least one region of interest; and c) ashear wave imaging step comprising: c1) an excitation substep ofgenerating the shear wave inside the anisotropic medium based on saidset of spatial characteristics, the shear wave being generated byemitting at least one focused ultrasound wave in the anisotropic mediumusing an array of transducers controlled by a central processor, thelocation of focal points of said at least one focused ultrasound wavesand the timing of said at least one focused ultrasound waves beingdetermined by the central processing unit based on the extracted set ofspatial characteristics of the anisotropic medium, the location of thefocal points and the timing of the plurality of focused ultrasound wavesgenerating the shear wave being determined so that a wave front of saidshear wave is substantially perpendicular to at least one of: adirection of detected anisotropic features in the anisotropic medium,and a selected excitation spatial direction in the anisotropic medium,c2) an observation substep of observing the propagation of said shearwave simultaneously at a multitude of points in the observation field.2. The method according to claim 1, wherein the set of spatialcharacteristics of the anisotropic medium further comprises at least oneof the following: a direction, spatial angle or spatial position ofanisotropic features in the at least one region of interest, and aspecific excitation spatial direction in the at least one region ofinterest.
 3. The method according to claim 1, wherein the initialphysical parameter is an image of the at least one region of interest inthe anisotropic medium acquired using B-mode ultrasonic imaging.
 4. Themethod according to claim 1, wherein the initial ultrasonic acquisitionstep comprises an initial shear wave imaging step comprising: a1) anexcitation substep of generating an initial shear wave is generatedinside the anisotropic medium with at least one shear wave direction,and a2) an observation substep of observing the initial shear wavesimultaneously at a multitude of points in the at least one region ofinterest to acquire an initial physical parameter.
 5. The methodaccording to claim 4, wherein the initial ultrasonic acquisition stepcomprises a plurality of shear wave imaging steps associated with aplurality of shear wave directions and with a plurality of initialphysical parameters acquired in at least one region of interest in theanisotropic medium, each shear wave imaging step of the plurality ofshear wave imaging steps comprising: a1) an excitation substep ofgenerating inside the anisotropic medium with an associated shear wavedirection of the plurality of shear wave directions; and a2) anobservation substep of observing the propagation of said respectiveshear wave simultaneously at a multitude of points in the at least oneregion of interest to acquire an associated initial physical parameterof the plurality of initial physical parameters.
 6. The method accordingto claim 4, wherein said at least one initial physical parameter isimage data of the region of interest in the anisotropic medium acquiredusing shear wave imaging.
 7. The method according to claim 4, whereinsaid at least one initial physical parameters parameter is at least oneshear wave propagation parameter, acquired in the at least one region ofinterest, using shear wave imaging.
 8. The method according to claim 7,wherein said at least one shear wave propagation parameter is selectedfrom shear wave speed, shear modulus μ, Young's modulus E, shearelasticity μ1, shear viscosity μ2.
 9. The method according to claim 1,wherein during the initial ultrasonic acquisition step a), at least twoinitial physical parameters are acquired, respectively associated to atleast two distinct regions of interest in the anisotropic medium;wherein during the spatial characterization step b), at least two setsof spatial characteristics are determined, respectively based on the atleast two initial physical parameters, and respectively associated tothe at least two distinct regions of interest in the anisotropic medium,and wherein during the shear wave imaging step c), during the excitationsubstep c1), at least two shear waves are generated inside theanisotropic medium, respectively based on the at least two sets ofspatial characteristics, and during the observation substep c2), thepropagation of said at least two shear waves is observed simultaneouslyat a multitude of points in the observation field.
 10. The methodaccording to claim 1, wherein the spatial characterization step b)comprises extracting the set of spatial characteristics by performingfeatures detection on at least one image of the anisotropic mediumacquired during the initial ultrasonic acquisition step a).
 11. Themethod according to claim 1, wherein the spatial characterization stepb) comprises comparing shear wave propagation parameters of theplurality of shear wave propagation parameters acquired during theinitial ultrasonic acquisition step a) to determine a preferredexcitation spatial direction in the anisotropic medium.
 12. The methodaccording to claim 1, wherein the spatial characterization step b)comprises displaying an image of the anisotropic medium acquired duringthe initial ultrasonic acquisition step a) to a user using a displaydevice connected to a central processing unit, said user indicatingspatial characteristics of the anisotropic medium using an input deviceconnected to said central processing unit.
 13. The method according toclaim 12, wherein said user indicates the spatial characteristics of theanisotropic medium by moving, using said input device, a position of avirtual line displayed above said image of the anisotropic medium onsaid display device, said line being indicative of a spatialcharacteristic of the spatial characteristics of the anisotropic medium.14. The method according to claim 12, wherein said user indicates thespatial characteristics by measuring, on said image of the anisotropicmedium displayed on the display device, a numerical value of a spatialcharacteristic of the anisotropic medium using conventional anglemeasurement tools provided by an ultrasound system, said user thenentering said numerical value in the central processing unit using theinput device.
 15. The method according to claim 1, wherein the locationof the focal points and the timing of the plurality of focusedultrasound waves generating the shear wave are determined so that apropagation direction of said shear wave is substantially aligned withat least one of: a direction of anisotropic detected features in theanisotropic medium, and a selected excitation spatial direction in theanisotropic medium.
 16. The method according to claim 1, wherein theobservation substep c2) comprises the operations of: c2-1) causing anarray of transducers that are controlled independently of one another toemit into the anisotropic medium a succession of ultrasound waves withspatial coverage and timing configured so that said ultrasound wavesexhibit at least partial spatial and temporal overlap with thepropagating shear wave in the observation field, and c2-2) causing soundsignals received from the anisotropic medium to be detected and recordedin real time by said array of transducers, said signals comprisingechoes generated by the ultrasound waves interacting with scatterers insaid anisotropic medium, the shear wave imaging step c) furthercomprising at least one processing substep c3) during which: c3-1) thesound signals received successively from the anisotropic medium duringoperation c2-2) are processed in order to determine successivepropagation images of the shear wave, and c3-2) at least one movementparameter of the anisotropic medium is determined at different points ofthe observation field.
 17. The method according to claim 16, wherein,during the initial ultrasonic acquisition step, an initial shear wavepropagating along at least two shear wave directions is generated insidethe anisotropic medium, and during the processing substep, sound signalsreceived from the anisotropic medium are filtered according to the atleast two shear wave directions to determine said at least one initialphysical parameter.
 18. The method according to claim 16, wherein saidmovement parameter is a displacement of the anisotropic medium.
 19. Themethod according to claim 16, wherein at said operation c2-1), saidunfocused ultrasound compression waves are emitted at a rate of at least300 shots per second.
 20. The method according to claim 16, wherein thefocused ultrasound wave emitted during excitation substep c1) presents afrequency f lying in the range 0.1 MHz to 100 MHz, and is emitted for aduration of k/f seconds, where k is an integer lying in the range 50 to5000 and f is expressed in Hz.
 21. The method according to claim 16,wherein the focused ultrasound wave emitted during excitation substepc1) presents a frequency lying in the range 0.5 MHz to 15 MHz and isemitted during a succession of emission periods separated by restperiods, the emission periods following one another at a rate lying inthe range 10 to 1000 emissions per second.
 22. The method according toclaim 16, wherein the focused ultrasound wave emitted during excitationsubstep c1) is a linear combination of two monochromatic signals havingrespective frequencies f1 and f2 such that 20 Hz≤|f1−f2|≤1000 Hz. 23.The method according to claim 16, wherein a focused ultrasound waveemitted during excitation substep c1) is focused simultaneously on aplurality of focal points.
 24. The method according to claim 16, whereinthe image processing substep c3) is followed by a mapping substep c4)during which, based on a variation in the movement parameter over time,at least one shear wave propagation parameter is calculated at at leastsome points of the observation field in order to determine a map of saidpropagation parameter in the observation field.
 25. The method accordingto claim 24, wherein the shear wave propagation parameter which iscalculated during the mapping substep c4) is selected from shear wavespeed, shear modulus, Young's modulus, shear wave attenuation, shearelasticity, shear viscosity, mechanical relaxation time, and the inverseof local strain.
 26. The method according to claim 24, wherein thesubsteps c1) to c4) are repeated successively while emitting differentpluralities of focused ultrasound waves during successive excitationsubsteps c1), and then combining the maps obtained during the successivemapping substeps c4) in order to calculate a combination map of theobservation field.
 27. The method according to claim 24, wherein thesteps b) and c) are reiterated at least once, a map of a propagationparameter in the observation field acquired during step c) at iterationn being used as the initial physical parameter for step b) at iterationn+1.
 28. An imaging apparatus for implementing the shear waveelastography method according to claim 1 for imaging an observationfield in an anisotropic medium, the apparatus comprising: an array oftransducers that are controlled independently of one another by at leastone electronic central unit configured to: acquire the at least oneinitial physical parameter in at least one region of interest in theanisotropic medium, determine the set of spatial characteristics of theanisotropic medium based on the initial physical parameter, said set ofspatial characteristics comprising a direction or a spatial angle of ananisotropic feature in the at least one region of interest, cause theshear wave to be generated inside the anisotropic medium based on saidset of spatial characteristics, and observe the propagation of saidshear wave simultaneously at a multitude of points in the observationfield.